1. Field of the Invention
The present invention relates to a light emitting device having a vertical structure, and more particularly, to a light emitting device having a vertical structure and a method for manufacturing the same which are capable of increasing light extraction efficiency.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well known as a semiconductor light emitting device which converts current to light, to emit light. Since a red LED using GaAsP compound semiconductor was commercially available in 1962, it has been used, together with a GaP:N-based green LED, as a light source in electronic apparatuses, for image display.
The wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. This is because the wavelength of the emitted light depends on the band gap of the semiconductor material representing energy difference between valence-band electrons and conduction-band electrons.
Gallium nitride (GaN) compound semiconductor has been highlighted in the field of high-power electronic devices because it exhibits a high thermal stability and a wide band gap of 0.8 to 6.2 eV. One of the reasons why GaN compound semiconductor has been highlighted is that it is possible to fabricate a semiconductor layer capable of emitting green, blue, or white light, using GaN in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, it is possible to adjust the wavelength of light to be emitted, using GaN in combination with other appropriate elements. Accordingly, where GaN is used, it is possible to appropriately determine the materials of a desired LED in accordance with the characteristics of the apparatus to which the LED is applied. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED to replace a glow lamp.
On the other hand, initially-developed green LEDs were fabricated using GaP. Since GaP is an indirect transition material causing degradation in efficiency, the green LEDs fabricated using this material cannot practically produce light of pure green. By virtue of the recent success of growth of an InGaN thin film, however, it has been possible to fabricate a high-luminescent green LED.
By virtue of the above-mentioned advantages and other advantages of GaN-based LEDs, the GaN-based LED market has rapidly grown. Also, techniques associated with GaN-based electro-optic devices have rapidly developed since the GaN-based LEDs became commercially available in 1994.
GaN-based LEDs have been developed to exhibit light emission efficiency superior over that of glow lamps. Currently, the efficiency of GaN-based LEDs is substantially equal to that of fluorescent lamps. Thus, it is expected that the GaN-based LED market will grow significantly.
Despite the rapid advancement in technologies of GaN-based semiconductor devices, the fabrication of GaN-based devices suffers from a great disadvantage of high-production costs. This disadvantage is closely related to difficulties associated with growing of a GaN thin film (epitaxial layer) and subsequent cutting of finished GaN-based devices.
Such a GaN-based device is generally fabricated on a sapphire (Al2O3) substrate. This is because a sapphire wafer is commercially available in a size suited for the mass production of GaN-based devices, supports GaN epitaxial growth with a relatively high quality, and exhibits a high processability in a wide range of temperatures.
Further, sapphire is chemically and thermally stable, and has a high-melting point enabling implementation of a high-temperature manufacturing process. Also, sapphire has a high bonding energy (122.4 Kcal/mole) and a high dielectric constant. In terms of a chemical structure, the sapphire is a crystalline aluminum oxide (Al2O3).
Meanwhile, since sapphire is an insulating material, available LED devices manufactured using a sapphire substrate (or other insulating substrates) are practically limited to a lateral or vertical structure.
FIG. 1 shows a structure of an LED device having a lateral structure of the aforesaid general GaN-based LEDs.
A lateral type LED device includes an n-type GaN layer 2 formed on a sapphire substrate 1, an active layer 3 (light emitting layer) formed on the n-type GaN layer 2, and a p-type GaN layer 4 formed on the active layer 3. An n-type electrode 5 is formed on a surface of the n-type GaN layer 2, from which the active layer 3 is removed. A p-type electrode 6 is formed on the p-type GaN layer 4.
Recent researches in the GaN-based semiconductor light emitting devices are focused on the increase of luminance. Methods for increasing luminance of the light emitting devices include a method for improving internal quantum efficiency and a method for improving light extraction efficiency. Recently, researches in the method for improving the light extraction efficiency have been actively proceeded.
The representative methods for increasing the light extraction efficiency include a method of etching the sapphire substrate with a regular pattern, roughening a surface of the p-type GaN layer, and forming a photonic crystal with a constant period by etching the p-type GaN layer.
At present, the methods of etching the sapphire substrate and roughening the surface of the p-type GaN layer are applied to the mass production technologies of the light emitting devices. The method using the photonic crystal has been theoretically well known and studied through laboratorially simulated experiment. However, the method using the photonic crystal is not applied to the mass production technologies of the light emitting devices until now.
The method using the photonic crystal has superior light extraction efficiency to the methods of etching the sapphire substrate and roughening the surface of the p-type GaN layer.
As shown in FIG. 2, the representative method using the photonic crystal is to form a photonic crystal 7 by etching the p-type GaN layer 4 with a constant periodical pattern in the basic structure of the LED device depicted in FIG. 1.
However, this method has a limitation in the improvement of the light extraction efficiency, because of basically low electrical features of the p-type GaN layer 4, a thin thickness and degradation of the electrical features by the etching.
Another method is to use a structure that the p-type GaN layer is grown on the substrate, the light emitting layer is grown on the p-type GaN layer 2 and the n-type GaN layer is grown on the light emitting layer, and to form a photonic crystal structure on the n-type GaN layer.
However, basically low electric conductivity of the p-type GaN layer, low crystalline quality and degradation of electrical features by the etching make the growth of the p-type GaN layer on the substrate impossible.
Another method is to grow the n-type GaN layer on the sapphire substrate, to subsequently grow the light emitting layer and the p-type GaN layer, and then to grow the n-type GaN layer again. This method is to use electric tunnel junction characteristics between the p-type GaN layer and the n-type GaN layer.
However, because of low electrical features of the p-type GaN layer, this method also has problem that resistance at a junction portion is increased and resultantly operating voltage of the device is increased.
Yet another method is to sequentially grow the n-type GaN layer, the light emitting layer and the p-type GaN layer over the sapphire substrate, to bond a reflective layer and a metal plate having a good heat dissipating effect, and to form the photonic crystal by etching the exposed surface of the n-type GaN layer, from which the sapphire substrate is removed.
However, because the metal plate is not stable sufficiently in the etching process of the thin film layer, it is difficult to perform the etching process, and productivity is low.